1. Field of the Invention
The present invention relates to a sewing machine which forms stitches by utilizing the cooperation of a needle bar and a hook or shuttle (i.e., thread-loop catching device), and in particular to such a sewing machine which includes respective motors for driving a needle bar and a hook.
2. Related Art Statement
A sewing machine forms stitches by utilizing the cooperation of a needle bar which is reciprocated when an upper shaft is rotated and a hook which is rotated together with a lower shaft. In the sewing machine, the needle bar is moved downward so that a needle thread is conveyed into a sewing bed and, when the needle bar is moved upward, the hook is rotated to catch the needle thread, so that a stitch is formed in a material being sewn, such as cloth or leather. The sewing machine may be of a type which includes an upper shaft which is directly connected to an electric motor and a lower shaft which is connected to the upper shaft via pulleys, a belt, etc. so that the rotation of the upper shaft is transmitted to the lower shaft. In this case, since the upper and lower shafts are mechanically connected to each other, the two shafts are rotated in synchronism with each other. Accordingly, good stitches are produced.
Recently, there has been proposed a sewing machine which includes respective electric motors for rotating an upper and a lower shaft. This arrangement contributes to increasing the degree of freedom of machine design and/or the degree of compactness of machine construction. This sewing machine is disclosed in, e.g., Japanese Patent Application laid open for inspection under Publication No. 3(1991)-234292. In this second case, the respective rotations of the upper and lower shafts should be synchronized with each other. To this end, the sewing machine may include an encoder for detecting a rotation speed or phase of a needle-bar drive motor and a control device for controlling a hook drive motor based on the detected rotation speed or phase of the needle-bar drive motor. This sewing machine is disclosed in, e.g., Japanese Patent Application laid open for inspection under Publication No. 7(1995)-657.
However, the above-indicated second sewing machine suffers from the following problem concerning the synchronization of respective rotations of the needle-bar and hook drive motors: When the sewing machine forms stitches in a lapped portion of material or sews varying plies or thicknesses of material, a resistance exerted to a sewing needle secured to the needle bar increases, so that the rotation speed of the needle-bar drive motor decreases. In this case, the control device controls the hook drive motor so that the rotation speed thereof decreases as the rotation speed of the needle-bar drive motor does. However, if the sewing machine is operated at a high speed, i.e., the needle bar is reciprocated at a high speed, then the hook drive motor may not follow up the decreasing of rotation speed of the needle-bar drive motor, so that the hook drive motor may not be rotated in synchronization with the rotation of the needle-bar drive motor and the hook may even fail to catch the loop of a needle thread conveyed by the sewing needle secured to the needle bar. Thus, good stitches may not be produced in the work sheet.